just one more thought to add... even though my kids aren't autistic, they were (and are) major climbers and not so good in the "listen to mom and dad" department. my ds is 5 (he'll be 6 in the summer) and he's part spiderman, climbing up our walls in the house. we put in a pool last year, and there was no fence tall enough to enclose the pool we felt safe with. he could (and would) climb a 20 foot fence easily. we opted for an automatic pool cover instead, and it was worth every penny. i've had people say "just tell him don't climb the fence, etc, etc", but i know my kid, and a fence just wasn't a safe option for us. we've also put nails on all the upstairs windows so the windows can only open a few inches (but if a child pushed hard enough, they would open, and an adult could easily open them. have to consider fire safety too... hard line to walk). my point is, you know your child best, and not only for his safety, but for your own piece of mind, i'd just go w/ the ground floor, because as he gets older, he'll figure out how to open the balcony doors, etc, but still might not have the safety sense he needs to stay safe (again, my almost 6yo doesn't have an ounce of it yet!) hope it comes soon! oh, and you're obviously already doing what's best for your child... just wanted to say how great that is of you!
